The present invention relates to a suspension of a vehicle, and in particular relates to a torsion-beam type of suspension provided with a torsion beam which is connected to a pair of right-and-left trailing arms.
Conventionally, a torsion-beam type of suspension, which comprises a pair of right-and-left trailing arms, front end portions of which are pivotally supported at a vehicle body below a floor panel and rear end portions of which rotatably support a pair of right-and-left wheels and a torsion beam extending in a vehicle width direction, both end portions, in the vehicle width direction, of which are connected to the pair of right-and-left trailing arms, is known. In the torsion-beam type of suspension, the torsion beam which is comprised of an open cross-section member which includes a pair of half portions (a one-side half portion and an other-side half portion) extending in its longitudinal direction and has a U-shaped cross section or the like performs the functions of a torsion bar as well as a spring. This type of suspension has a simple structure and a small number of parts, so it is advantages in terms of costs and layout space. Thus, the torsion-beam type of suspension has been widely applied to a rear suspension of F-F (front-engine front-drive) vehicles.
In vehicles, such as F-R (front-engine rear-drive) vehicles or four-wheel drive vehicles, in which rear wheels are driven, a drive force of an engine installed at a vehicle-body front portion is transmitted to a differential gear device installed at a vehicle-body rear portion via a propeller shaft extending longitudinally at a central portion, in a vehicle width direction, of the vehicle. Accordingly, it is required that the above-described torsion beam is configured to project upward at its central portion in order to avoid any interference with the propeller shaft. Meanwhile, as shown in FIG. 10, in the torsion-beam type of suspension, the wheel supported by this suspension swings with a theoretical swing axis which corresponds to a straight line A which interconnects a shear center C positioned behind the central portion of a torsion beam B and a front end portion F of the trailing arm. Therefore, in a case in which the central portion, in the vehicle width direction, of the torsion beam B curves upward, the shear center C also moves upward according to this curving of the torsion beam B. Consequently, there occurs a problem in that the alignment-change characteristics of the suspension may change improperly.
A suspension disclosed in Japanese Patent Laid-Open Publication No. H4-283114 is configured, in order to restrain the torsion beam's sheer center from moving upward, such that a torsion beam has a curve portion curing upward at a central portion, in the vehicle width direction, thereof and an open cross-section of the torsion beam opens upward, so that the shear center of the torsion beam is located at the level below the torsion beam. In general, a long member, such as the torsion beam, has a tendency that in a case in which it has a bending portion at some point in its longitudinal direction, a cross-section crumbling happens to this bending portion of the long member, so that the rigidity decreases. Therefore, there is a problem in that if the upward curve portion is formed at the central portion, in the vehicle width direction, of the torsion beam, the rigidity of the curve portion of the torsion beam may decrease improperly, so that the vehicle may be inferior in the maneuverability and stability.
A suspension disclosed in Japanese Patent Laid-Open Publication No. H8-324218 is configured such that a torsion beam comprises a curve-shaped central portion which projects upward and a pair of straight-shaped portions which are smoothly continuous from both end portions of the curve-shaped central portion via curve portions having the radius of curvature which changes continuously from that of the curve-shaped central portion. Herein, a large amount of bending moment is generated at the straight-shaped portions located at the both end portions of the torsion beam and a small amount of bending moment is generated at the curve-shaped central portion, so that the rigidity of the torsion beam can be improved.
When a large external force acts on a wheel, this external force is transmitted via the trailing arm such that a vertical load of that is transmitted to a shock absorber and the like and a torsional load and a lateral force of that is transmitted to the torsion beam. The torsional load and the lateral force are absorbed by bending of the torsion beam itself and closing deformation of the open cross-section which is formed by the one-side half portion and the other-side half portion. Herein, in order that the torsion beam ensures the torsional rigidity and the rigidity against the lateral force, a crushed pipe type of torsion beam, in which the torsion beam which is made of a cylindrical pipe member is crushed (pressed) from a direction perpendicular to an axis of the pipe member over its longitudinal direction so as to have a cross section formed in a V or U shape with a peak portion, is also known. The rigidity of this torsion beam can be changed by changing the size of a hollow portion formed by both end portions in the cross section of the torsion beam. A torsion beam disclosed in Japanese Patent Laid-Open Publication No. 2005-306177 is configured to have the cross section formed in the V or U shape with the peak portion, the hollow portion formed by the both end portions in the cross section, and a gap formed between a pair of one-side half portion and other-side half portion of the torsion beam which expands continuously from the peak portion interconnecting the both half portions toward the both end portions.
The suspension of the above-described first patent document can restrain the sheer center of the torsion beam from moving upward, so that changing of the alignment-change characteristics of the suspension can be prevented when the wheel is in a bound state or in a rebound state. However, the suspension of the first patent document has a problem in that the productivity of the torsion beam may deteriorate. That is, since the torsion-beam type of suspension is configured such that the open cross-section formed by the one-side half portion and the other-side half portion which respectively extend in the longitudinal direction of the torsion beam opens upward, there is a concern that water or the like splashed by the wheels may stay inside the open cross-section of the torsion beam. Therefore, it may be required that some holes for draining such water or the like are formed at the peak portion of the torsion beam. In a case in which the crushed pipe type of torsion beam disclosed in the above-described third patent document is used, it may be necessary that anti-rusting treatment or the like are applied to the hollow portion, so that forming of such drain holes may become uneasy.
The suspension of the above-described second patent document enables welding of a reinforcing member by configuring the torsion beam such that the both end portions, in the vehicle width direction, thereof are straight-shaped, so that the rigidity and the durability of the suspension can be improved. However, the suspension of the second patent document may not be sufficient in rigidity improvement of the torsion beam. That is, since the curve-shaped central portion and the straight-shaped both-end portions are continuous via the curve portions, an axial center of the curve-shaped central portion and an axial center of the straight-shaped end portion cross each other at a specified angle, so that a stress concentration portion where a stress concentrates is formed at the curve portion. Therefore, the rigidity of the curve portion relatively decreases, comparing with the other portion of the torsion beam. Thus, the suspension of the first patent document may have the same problem.
Meanwhile, low-floor vehicles in which a floor panel is located at a low level are desired from aspects of a wide space, in a vertical direction, of a vehicle compartment or an easy access (ingress/egress) of passengers or an easy loading/unloading of baggage. However, since the suspensions of the above-described first or second patent documents have the upward-curve portion at the central portion, in the vehicle width direction, of the torsion beam and therefore this upward-curve portion possibly interferes with the floor panel when the vehicle is in the full-rebound state, there is a concern that the floor of the vehicle may not be positioned at a sufficiently low level.